1. Field of the Invention
The present invention relates to an apparatus for manufacturing a semiconductor device having a circuit structured with a thin film transistor. For example, the invention relates to an apparatus for manufacturing an electro-optical device, typically a liquid crystal display device, and the structure of electric equipment mounted with such an electro-optical device as a component. The present invention also relates to a method of fabricating the apparatus. Note that throughout this specification, the semiconductor device indicates general devices that may function by use of semiconductor characteristics, and that the above electro-optical device and electric equipment are categorized as the semiconductor device.
2. Description of the Related Art
In recent years, the technique of crystallizing and improving the crystallinity of a semiconductor film formed on an insulating substrate such as a glass substrate by laser annealing, has been widely researched. Silicon is often used as the above semiconductor film.
Comparing a glass substrate with a quartz substrate, which is often used conventionally, the glass substrate has advantages of low-cost and great workability, and can be easily formed into a large surface area substrate. This is why the above research is performed. Also, the reason for preferably using laser annealing for crystallization resides in that the melting point of a glass substrate is low. Laser annealing is capable of imparting high energy only to the semiconductor film without causing much change in the temperature of the substrate.
The crystalline semiconductor film is formed from many crystal grains. Therefore, it is called a polycrystalline semiconductor film. A crystalline semiconductor film formed by laser annealing has high mobility. Accordingly, it is actively used in, for example monolithic type liquid crystal electro-optical devices where thin film transistors (TFTs) are formed using this crystalline semiconductor film and fabricate TFTs for driving pixels and driver circuits formed on one glass substrate.
Furthermore, a method of performing laser annealing is one in which a pulse laser beam emitted from a excimer laser or the like, is processed by an optical system so that the laser beam thereof becomes a linear shape that is 10 cm long or greater or a square spot that is several cm square at an irradiated surface to thereby scan the laser beam (or relatively move the irradiation position of the laser beam to the irradiated surface). Because this method is high in productivity and industrially excellent, it is being preferably employed.
Different from when using a spot shape laser beam which requires a front, back, left, and right scan on an irradiated surface, when using the linear beam, the entire irradiated surface can be irradiated by the linear beam which requires only scanning at a right angle direction to the linear direction of the linear beam, resulting in the attainment of a high productivity. To scan in a direction at a right angle to the linear direction is the most effective scanning direction. Because a high productivity can be obtained, using the linear beam which is linear in the irradiated surface that is emitted from the pulse oscillation type excimer laser and processing it into a linear beam by an appropriate optical system for laser annealing at present is becoming mainstream.
Shown in FIG. 1 is an example of the structure of an optical system for linearizing the shape of a laser beam on the irradiated surface. This structure is a very general one and all aforementioned optical systems conform to the structure of the optical system shown in FIG. 1. This structure of the optical system not only transforms the shape of the laser beam in the irradiated surface into a linear shape, but also homogenizes the energy of the laser beam in the irradiated surface at the same time. Generally, an optical system that homogenizes the energy of a beam is referred to as a beam homogenizer.
If the excimer laser, which is ultraviolet light, is used as the light source, then the core of the above-mentioned optical system may be preferably made of, for example, entirely quartz. The reason for using quartz resides in that a high transmittance can be obtained. Further, it is preferable to use a coating in which a 99% or more transmittance can be obtained with respect to a wavelength of the excimer laser that is used.
The side view of FIG. 1 will be explained first. Laser beam emitted from a laser oscillator 101 is split at a right angle direction to the advancing direction of the laser beam by cylindrical lens arrays 102a and 102b. The direction is referred to as a longitudinal direction throughout the present specification. When a mirror is placed along the optical system, the laser beams in the longitudinal direction will curve in the direction of light curved by the mirror. These laser beams in this structure are split into 4 beams. The split laser beams are then converged into 1 beam by a cylindrical lens 104. Then, the converged laser beam are split again and reflected at a mirror 107. Thereafter, the split laser beams are again converged into 1 laser beam at an irradiated surface 109 by a doublet cylindrical lens 108. A doublet cylindrical lens is a lens that is constructed of 2 pieces of cylindrical lenses. Thus, the energy in the width direction of the linear laser beam is homogenized and the length of the width direction of the linear beam is also determined.
The top view of FIG. 1 will be explained next. Laser beam emitted from the laser oscillator 101 is split at a right angle direction to the advancing direction of the laser beam and at a right angle direction to the longitudinal direction by a cylindrical lens array 103. The right angle direction is called a vertical direction throughout the present specification. When a mirror is placed along the optical system, the laser beams in the vertical direction will curve in the direction of light curved by the mirror. The laser beams in this structure is split into 7 beams. Thereafter, the split laser beams are converged into 1 beam at the irradiated surface 109 by the cylindrical lens 108. Thus, homogenization of the energy in the longitudinal direction of the linear beam is made and the length of the longitudinal direction is also determined.
The above lenses are made of synthetic quartz for correspondence to excimer laser. Furthermore, coating is implemented on the surfaces of the lenses so that the excimer laser will be well transmitted. Therefore, the transmittance of excimer laser through each lens is 99% or more.
By irradiating the linear beam linearized by the above structure of the optical system in an overlapping manner with a gradual shift in the width direction thereof, laser annealing is implemented to the entire surface of a semiconductor film to thereby crystallize the semiconductor film and thus its crystallinity can be enhanced.
A typical method of manufacturing a semiconductor film that is to become the object to be irradiated is shown next. First, for example, a 5 inch square Corning 1737 substrate having a thickness of 0.7 mm is prepared as the substrate. Then a 200 nm-thick SiO2 film (silicon oxide film) is formed on the substrate and a 50 nm-thick amorphous silicon film is formed on the surface of the SiO2 film. The substrate is exposed under an atmosphere containing nitrogen gas at a temperature of 500xc2x0 C. for 1 hour to thereby reduce the hydrogen concentration in the film. Accordingly, the laser resistance in the film is remarkably improved.
The XeCl excimer laser L3308 (wavelength: 308 nm, pulse width: 30 ns) manufactured by Lambda Co. is used as the laser apparatus. This laser apparatus generates a pulse oscillation laser and has the ability to output an energy of 500 mJ/pulse. The size of the laser beam at the exit of the laser beam is 10xc3x9730 mm (both half-width). Throughout the present specification, the exit of the laser beam is defined as the perpendicular plane in the advancing direction of the laser beam immediately after the laser beam is emitted from the laser irradiation apparatus.
The shape of the laser beam generated by the excimer laser is generally rectangular and is expressed by an aspect ratio which falls under the range of the order of 3 to 5. The intensity of the laser beam grows stronger towards the center of the beam and indicates the Gaussian distribution. The size of the laser beam processed by the optical system having the structure shown in FIG. 1 is transformed into a 125 mmxc3x970.4 mm linear beam having a uniform energy distribution.
When irradiating a laser beam to the above-mentioned semiconductor film, the most suitable overlapping pitch is approximately {fraction (1/10)} of the beam width (half-width) of the linear beam. The uniformity of the crystallinity in the film is thus improved. In the above example, the half-width of the linear laser beam was 0.4 mm, and therefore the pulse frequency of the excimer laser was set to 30 hertz and the scanning speed was set to 1.0 mm/s during irradiation using the laser beam. At this point, the energy density in the irradiated surface of the laser beam was set to 420 mJ/cm2. The method described so far is a very general method employed for crystallizing a semiconductor film using a linear beam.
When an extremely attentive observation is made to a semiconductor film that has been laser annealed by using the above-mentioned linear beam, very faint interference patterns were seen in the film. The cause of the interference patterns seen in the film resides in that the laser beam is split and assembled in one region, and therefore the split light brings about interference with each other. The coherent length of the excimer laser is about several xcexcm to several tens of xcexcm.
When a laser annealing is performed to a semiconductor film by using a conventional optical system, since faint interference is seen, an optical system in which interference is suppressed is provided according to the present invention. Besides, although the conventional optical system is complicated as shown in FIG. 1, according to the present invention, a uniform laser beam can be obtained by an optical system mainly including a beam collimator for transforming a laser beam into a parallel light and a reflecting mirror, and it is also possible to simplify the optical system for forming the laser beam having a linear or rectangular shape on an irradiation surface.
A laser beam has a feature that even if laser beams are emitted from the same light source, they do not interfere with each other if there is an optical path difference equal to or longer than a coherent length. First, an explanation will be made with reference to FIG. 2. FIG. 2 shows a laser oscillator 201, a half mirror 202, mirrors 203 to 205, and an irradiation surface 206. It is assumed that the distance between the half mirror 202 and the mirror 203 is equal to the distance between the mirror 204 and the mirror 205, and the distance between the mirror 203 and the irradiation surface 206 is equal to the distance between the mirror 205 and the irradiation surface 206. A laser beam emitted from the laser oscillator 201 is divided by the half mirror 202 into a laser beam transmitted through the half mirror and traveling straight and a laser beam traveling in a direction perpendicular to the original traveling direction. After the laser beam bent to the perpendicular direction is reflected by the mirror 203, it reaches the irradiation surface 206. On the other hand, the laser beam transmitted through the half mirror 202 reaches the irradiation surface 206 through the mirrors 204 and 205.
In this way, although the laser beams divided by the half mirror in the two directions are assembled again to make one beam at or in the vicinity of the irradiation surface 206, the distance from the half mirror 202 to the mirror 204 is an optical path difference of the laser beams divided in two. In the case where this optical path difference is longer than the coherent length of the laser beam, interference at the irradiation surface does not occur.
For example, in the case where an excimer laser is used as the laser oscillator 201, since the coherent length of the excimer laser is about several xcexcm to several tens of xcexcm, if the distance from the half mirror 202 to the mirror 204 is several mm, the interference does not occur at the irradiation surface 206. In the case where a YAG laser is used as the laser oscillator 201, since the coherent length of the YAG laser is longer than the coherent length of the excimer laser, if the distance from the half mirror 202 to the mirror 204 is made longer than that in the case of the excimer laser, the interference at the irradiation surface 206 does not occur.